In the realm of body exercise and strength training, there is a need to strengthen the muscles of the thigh which are used to flex the leg at the knee. In order to build strength in the user's leg flexion muscles, it is essential to increase the resistive load on the muscles as the user's strength level increases. It is also desirable to reduce the risk of injury and to increase the range of motion as the exercise is performed.
Various exercise devices have been devised to exercise the thigh muscles as the legs are flexed at the knee. These devices function by allowing the user, while lying prone, to engage the posterior lower leg underneath a weighted bar and then to force the bar in an upward arc as the leg is flexed. Some of these devices cause the effective resistance to maximize as the knee reaches full flexion when the knee is most vulnerable to injury. Other leg flexion exercisers work on similar principles but with complicated lever arrangements without satisfactorily reducing the resistance on the leg as the leg is flexed at the knee.